The three-electrode gas filled surge arrester is now widely used in the protection of telephone and other networks and equipment; see for example U.S. Pat. No. 3,289,027; current commercial types are exemplified by TII types 21 and 31 manufactured and sold by the assignee herein, TII Corporation (formerly Telecommunications Industries, Inc.).
As is well known, the performance of gas filled arresters depends among other things on gap dimensions, nature of the gas, and gas pressure. Typically, for a striking voltage of ca 400 volts, gas pressure is approximately 45 mm while the gap dimension between each electrode and the centerbody is in the order of 0.070", and between the faces of the line electrodes is about 0.065".
As with technology in general, there have been continuous efforts to improve the performance of the three element gas tube arrester, encouraged to some extent by constantly increasing stringency in the various municipal and institutional codes and regulations governing these elements.
With respect to one test which must now be passed as a condition to U.L. listing for safety under abnormal fault conditions, the gas tube is subjected to a current of 120 amperes for 1.2 seconds (being the fusible time of a bridle wire or equivalent fusible element).
Tests have indicated that some designs may encounter difficulty in meeting this test, the casing occasionally rupturing and the rupture being accompanied by a spray of molten material. It has been discovered that the rupturing depends on the behavior of the electrodes during overload. If the electrodes sag sufficiently to touch the centerbody, rupture is usually averted; otherwise, rupture is likely. In the former case a short circuit is established; in the latter high heat generating resistance prevails.
In devising means to meet these more stringent tests, it is strongly preferred, if not essential, that current commercial designs be subject to as little change as possible since such designs constitute proven, widely accepted arresters which meet many codes and regulations and are dimensioned to fit existing commercial mountings.
Various design changes based on these constraints, including lengthening of the electrodes and changes in the cavity of the electrode carrying the pinch tube alone, have not been considered to be entirely satisfactory solutions.
In contrast, the incorporation of a sleeve coaxially telescoped inside the centerbody electrode (the term sleeve as used herein also contemplates a thickening of the centerbody wall in the discharge region) provides a successful solution to the housing rupture problem. This change reduces gap dimensions thus requiring a compensating increase in gas pressure to maintain the previous breakdown potential. While these changes could conceivably prejudice other performance specifications, it has been found to the contrary that the changes actually provide an improvement in d.c. breakdown voltage stability.
To understand this result, it is noted that in some cases degradation of d.c. breakdown voltage occurs after a certain number of high current strikes. The drop is the result of the deposition of metal on the surfaces of the ceramic insulator rings which separate the centerbody from the end caps. This deposition occurs as a consequence of sputtering of the electrode metal in the low pressure inert gas atmosphere of the tube. In the designs of interest, the ceramic/electrode spacing is less than the annular gaps; with the ceramic coated with metal a shorter gap is thus established causing a drop in striking voltage. Such a drop may cause the arrester to fire in the presence of normal working potentials or signals, e.g., ringing voltages.
With the addition of the sleeve, the reduced annular centerbody/electrode gap more nearly approaches the ceramic/electrode spacing such that the d.c. breakdown voltage of the latter (when repeated firing deposits metal on the ceramic) more nearly equals that of the former electrode gap breakdown. As a consequence, tubes incorporating the invention are able not only to meet more stringent overload specifications, but also to provide better stability in strike voltage to insure that after repeated operation under severe service conditions, the arrester will not be triggered by application of carrier power supply voltages and ringing potentials to the telephone lines.